Question About Usage

[unclepaul84]

Hello,

Let me apologize in advance for dumb question.

I am trying to figure out a cross-platform way to store and access structs via memory mapped files.

I guess the question is how do I perform operations with mio on types wider than byte or char?

Thanks in Advance

struct Tick
{    
   double Price;
  int Volume;
};

   const auto path = "ticks.bin";
 

      std::error_code error;
  
      mio::mmap_sink rw_mmap = mio::make_mmap_sink(path, 0, mio::map_entire_file, error);
  
      for(Tick *t = rw_mmap->begin(); t != rw_mmap->end(); ++t)
      {
          t->Price = 5; //
      } 

[vimpunk]

Hello @unclepaul84,

I'm thrilled that you're giving mio a go! Unfortunately serialization is not supported, mio serves as a thin wrapper around the platform memory mapping facilities, which also don't provide this feature, afaik.

This could be a feature on top of mio, a serialization framework, but I think it may be better suited as a separate library to keep separation of concerns clearer.

Happy to accept feedback, of course.

[privefl]

I would need this too (reading only). Any idea how to easily implement this on top of 'mio' for this int-double struct?

[privefl]

What I have currently:

struct indval {
  int i;
  double x;
};

std::error_code error;
mio::mmap_source ro_mmap;
ro_mmap.map(path, error);
const indval * data = reinterpret_cast<const indval*>(ro_mmap.data());
cout << data->i << " // " << data->x << std::endl;

[sykhro]

@privefl what you're doing is bound to break because of padding and alignment. you can use Boost.serialization or, if you prefer something smaller, cereal

[spudwa]

I'm doing exactly what @privefl suggested and it works. When the original file is created sizeof(indval) bytes is written to disk. This will include any padding bytes added by the compiler. It's just a binary blob of bytes. As long as you use the same padding the bytes on disk and in memory are the same. This is highly non portable of course

[privefl]

I've finally used two doubles, for safety.